The present invention relates to bonding materials having different coefficients of thermal expansions, and more particularly, to an improved process for bonding copper to tungsten.
One of the difficulties in the fabrication and use of bonded joints of dissimilar materials is accommodating the differences in their respective coefficients of thermal expansion (CTE). CTE is also often referred to as the coefficient of linear expansion, and these terms have interchangeable meaning. The difference in CTE's of bonded materials can result in stresses being exerted at the bondline during thermal cycling and after joinder at elevated temperatures. These stresses can reduce the service life, and even cause failure of the joints. The joint failures are especially prone to occur during cool-down from the initial joining temperature or in the course of heat cycles during service.
An inter-metal bonding system of interest in nuclear fusion research in particular is that of copper to tungsten. As illustrated in FIG. 1, in the nuclear fusion research milieu, the application of copper-tungsten bonds generally involves joining an exterior surface of a water-cooled copper alloy heat sink 10 to a tungsten tile 12, referred to occasionally herein as "armor," to protect the copper alloy material constituting the heat sink 10 from the sputtering erosion and occasionally extreme temperatures that can be generated by the reactor. The copper heat sink 10 has interior water channels 11 that permit water to flow through the heat sink 10 for heat exchange and cooling purposes. Previously, a metal joint 13 has been made between the copper alloy heat sink 10 and the tungsten tile 12 by brazing or direct diffusion bonding techniques. Namely, a single layer of mixed copper and tungsten material has been brazed or diffusion bonded in-between the copper alloy heat sink 10 and the tungsten tile 12. However, according to the HANDBOOK OF CHEMISTRY AND PHYSICS, 56th Ed., CRC Press, Inc., Cleveland, Ohio, 1975, p. D-173, the coefficient of thermal expansion (at 25.degree. C.) for tungsten is 4.5.times.10.sup.6 (.degree. C.).sup.-1 while the value for copper is 16.6.times.10.sup.6. This considerable difference in the respective CTE's of copper and tungsten, if not adequately addressed, can set the stage for potential joint failures due to thermal stresses exerted on the structures during bonding of these materials.
For instance, brazed copper-tungsten joints are relatively thin intervening connection structures between the copper and tungsten bodies. This short joint distance hinders the ability of the joint to effectively spread out strain differences arising from differences in the copper and tungsten CTE's, which, in turn, aggravates thermal stresses. Similarly, a diffusion bond formed between the copper heat sink 10 and tungsten tile 12 by hot pressing at high temperature and pressure also results in a relatively narrow joint region between the respective substrates. For instance, a vacuum brazed layer joint may only be on the order of approximately 125 .mu.m thick, while a direct diffusion bonded joint formed by hot pressing may be only approximately 25 .mu.m thick.